In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs). Radio link monitoring (RLM) is a mechanism for a UE to monitor the quality of a downlink (DL) for determining if the radio link is good enough to continue transmission. For example, the UE shall monitor the DL quality based on cell-specific reference signal (CRS) to detect the downlink radio link quality for the serving cell. The UE shall also compare the estimated DL quality to thresholds QOUT and QIN for monitoring the downlink radio link quality of the serving cell. In addition to RLM, the UE shall consider radio link failure (RLF) to be detected upon physical layer problems based on N310/N311/T310 mechanism, random access problem indication from MAC layer, and indication from RLC layer that the maximum number of retransmission has been reached. Once RLF is detected, the UE gathers and stores RLF information and attempts RRC connection reestablishment. If such attempt fails, the UE goes back to RRC_IDLE. The UE may indicate the availability of the RLF report to eNB and report the RLF information to eNB upon request after successful RRC connection reestablishment or RRC connection setup.
In LTE Rel-10, the concept of carrier aggregation (CA) has been introduced to enhance the system throughput. With CA, two or more CCs are aggregated to support wider transmission bandwidth up to 100 MHz. A Rel-10 UE with reception and/or transmission capabilities for CA can simultaneously receive and/or transmit on multiple CCs corresponding to multiple serving cells. When CA is configured, the UE has only one RRC connection with the network. At RRC connection establishment/reestablishment or handover, one serving cell provides the NAS mobility information. At RRC connection reestablishment or handover, one serving cell provides the security input. This cell is referred to as the primary serving cell (PCELL), and other cells are referred to as the secondary serving cells (SCELLs). Depending on UE capabilities, SCELLs can be configured to form together with the PCELL as a set of serving cells.
RLM and RLF are only applied on PCELL in LTE Rel-10. RLF detection on DL PCELL is based on N310/N311/T310 mechanism. Deactivation and removal of DL SCELL suffering poor link quality should be under eNB command. Therefore, no autonomous UE deactivation and removal of such SCELL is allowed. Radio link monitoring (i.e., RLF physical layer problem detection based on N310/N311/T310) by the UE is not needed for DL SCELLs, because it is assumed that eNB can detect poor link quality e.g. from CQI reports and/or existing RRM measurement reports. Random access failure on UL PCELL triggers RRC connection reestablishment. UE never autonomously stops any transmission on an UL SCELL based on DL SCELL link quality.
In LTE rel-11, inter-band UL CA will be supported. In addition, various CA deployment scenarios will be supported, including inter-entity carrier aggregation, inter-eNB/intra-RAT carrier aggregation, and inter-eNB/inter-RAT carrier aggregation. For example, inter-entity carrier aggregation involves carrier aggregation through different transmission entities such as eNB with Remote Radio Header (RRH) and eNB with frequency selective repeaters. In these CA deployment scenarios, the UL timing advance values and the DL pathloss estimation for UL power control may be different from different paths, different entities, different frequency bands, and/or different RATs. For example, a UL SCELL may transmit through a different frequency band from UL PCELL and/or a UL SCELL may transmit through a different entity from UL PCELL. As a result, the timing advance and the pathloss on the SCELL could be very different from that of the PCELL.
In current state, RLM is only applied on PCELL, which is insufficient to determine if the radio link is good enough to continue transmission on SCELL. The RLF detection on physical layer problem is also only applied on PCELL, which cannot prevent interference to other users when SCELL is out-of-sync. For example, if radio link problem happens on DL SCELL used as a timing reference cell, it will cause incorrect UL timing so that inter symbol interference to other users may happen. Similarly, if radio link problem happens on DL SCELL used as a pathloss reference cell, it will cause incorrect pathloss estimation so that spurious UL SCELL transmission may happen and introduce interference to other users. Furthermore, the RLF detection upon random access problem is only applied on PCELL. Once the random access problem occurs on SCELL, the behavior is unclear. To prevent spurious and uncontrollable UL SCELL transmission and to monitor the random access on SCELL, a solution is sought.